Facile Synthesis of ZnO/WO3 Nanocomposite Porous Films for High-Performance Gas Sensing of Multiple VOCs

Volatile organic compounds (VOCs) in indoor environments have typical features of multiple components, high concentration, and long duration. The development of gas sensors with high sensitivity to multiple VOCs is of great significance to protect human health. Herein, we proposed a sensitive ZnO/WO3 composite chemi-resistive sensor facilely fabricated via a sacrificial template approach. Based on the transferable properties of self-assembled monolayer colloidal crystal (MCC) templates, two-dimensional honeycomb-like ordered porous ZnO/WO3 sensing matrixes were constructed in situ on commercial ceramic tube substrates with curved and rough surfaces. The nanocomposite thin films are about 250 nm in thickness with large-scale structural consistency and integrity, which facilitates characteristic responses with highly sensitivity and reliability. Furthermore, the nanocomposite sensor shows simultaneous responses to multiple VOCs that commonly exist in daily life with an obvious suppression sensing for traditional flammable gases. Particularly, a detection limit of 0.1 ppm with a second-level response/recovery time can be achieved, which is beneficial for real-time air quality assessments. We proposed a heterojunction-induced sensing enhancement mechanism for the ZnO/WO3 nanocomposite film in which the formation of abundant heterojunctions between ZnO and WO3 NPs significantly increases the thickness of the electron depletion layer in the bulk film and improves the formation of active oxygen species on the surface, which is conducive to enhanced responses for reducing VOC gases. This work not only provides a simple approach for the fabrication of high-performance gas sensors but also opens an achievable avenue for air quality assessment based on VOC concentration detection.


Introduction
Volatile organic compounds (VOCs) are mainly emitted from anthropogenic sources such as fossil fuel, paints, compressed aerosol, and biomass combustion, which have been recognized as critical precursors for tropospheric ozone and secondary organic aerosols (SOAs) [1][2][3][4]. Environmental problems generated by VOCs can also cause direct harm to respiratory, allergic, or immune systems of human beings, and even cancer in organs [5]. Nowadays, VOCs have been recognized as prominent hazards in the environmental atmosphere and have evolved into a global issue faced by all mankind [6]. Plenty of studies have shown that indoor building paints and household products can constantly release complex organic compounds, and their concentrations in indoor air may be an order of magnitude higher than those outdoors [7]. Obviously, this will cause longer and more serious damages to human health. Therefore, real-time monitoring of multiple VOCs and early warnings of air pollution have extreme significance in air quality assessment and human health protection [8]. At present, the identification and quantification of atmospheric VOCs mainly rely (Shanghai, China). Polystyrene (PS) sphere suspension (2.5 wt% in water, 500 nm in diameter) was obtained from Shanghai Huge Biotechnology Co., Ltd. (Shanghai, China). All the reagents were of analytical grade and used without further purification. Deionized water was produced in an ultrafilter system (Millipore Milli-Q system) with a resistivity of 18.2 MΩ·cm at 25 • C. Figure 1 schematically shows the direct fabrication of ZnO/WO 3 nanocomposite sensing films on ceramic tube substrates via a sacrificial template approach. Typically, the well-dispersed PS spheres were spread on the glass slide and self-assembled into hexagonally ordered arrays (Figure 1a) [32]. After natural drying, the monolayer colloidal crystal (MCC) was entirely transferred to the surface of the desired precursor solution and then re-transferred to the ceramic tube surface (Figure 1b,c). (NH 4 ) 6 H 2 W 12 O 40 ·XH 2 O and (CH 3 COO) 2 Zn·2H 2 O were mixed and dissolved in 50 mL distilled water in different proportions and stirred with a magnetic rotor for 10 min to obtain a homogeneous solution of the precursor so that the required precursor could infiltrate into the void between adjacent PS spheres of the MCC. After annealing at 400 • C for 2 h, the ZnO/WO 3 porous film was in situ obtained on the ceramic tube substrate due to the thermal decomposition of organic spheres and precursors (Figure 1d-f). By adjusting the relative content of (CH 3 COO) 2 Zn·2H 2 O, ZnO/WO 3 porous films with Zn/W atomic percentages of 3%, 5% and 10% were prepared and labeled as 3% ZnO/WO 3 , 5% ZnO/WO 3 , and 10% ZnO/WO 3 , respectively. The pure WO 3 ordered porous film was also prepared as a control.

Materials
Ammonium metatungstate ((NH4)6H2W12O40·5H2O) and zinc acetate dihydrate ((CH3COO)2Zn·2H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polystyrene (PS) sphere suspension (2.5 wt% in water, 500 nm in diameter) was obtained from Shanghai Huge Biotechnology Co., Ltd. (Shanghai, China). All the reagents were of analytical grade and used without further purification. Deionized water was produced in an ultrafilter system (Millipore Milli-Q system) with a resistivity of 18.2 MΩ·cm at 25 °C. Figure 1 schematically shows the direct fabrication of ZnO/WO3 nanocomposite sensing films on ceramic tube substrates via a sacrificial template approach. Typically, the well-dispersed PS spheres were spread on the glass slide and self-assembled into hexagonally ordered arrays (Figure 1a) [32]. After natural drying, the monolayer colloidal crystal (MCC) was entirely transferred to the surface of the desired precursor solution and then re-transferred to the ceramic tube surface (Figure 1b,c). (NH4)6H2W12O40·XH2O and (CH3COO)2Zn·2H2O were mixed and dissolved in 50 mL distilled water in different proportions and stirred with a magnetic rotor for 10 min to obtain a homogeneous solution of the precursor so that the required precursor could infiltrate into the void between adjacent PS spheres of the MCC. After annealing at 400 °C for 2 h, the ZnO/WO3 porous film was in situ obtained on the ceramic tube substrate due to the thermal decomposition of organic spheres and precursors (Figure 1d-f). By adjusting the relative content of (CH3COO)2Zn·2H2O, ZnO/WO3 porous films with Zn/W atomic percentages of 3%, 5% and 10% were prepared and labeled as 3% ZnO/WO3, 5% ZnO/WO3, and 10% ZnO/WO3, respectively. The pure WO3 ordered porous film was also prepared as a control. (c-e) The MCC was transferred intact to the precursor surface, followed by another transfer using a ceramic tube substrate. (f) ZnO/WO3 nanocomposite porous films were obtained by heat treatment at 400 °C for 2 h.

Characterization
The phase and crystal structures of the products were determined by X-ray diffraction (XRD) patterns, which were recorded with the X-ray diffractometer (the Philips X'Pert) with a 1D array detector using Cu Kα1 radiation (λ = 1.5406 Å). The morphology and structure of the product were observed by field emission scanning electron microscope (FE-SEM, FEI Sirion 200) and transmission electron microscope (TEM, JEOL JEM-2100). High-resolution transmission electron microscopic (HR-TEM) images and selected- Figure 1. Schematic diagram of template-assisted in situ preparation of ZnO/WO 3 nanocomposite porous sensing films. (a,b) A PS monolayer colloidal crystal (MCC) was prepared by interfacial self-assembly. (c-e) The MCC was transferred intact to the precursor surface, followed by another transfer using a ceramic tube substrate. (f) ZnO/WO 3 nanocomposite porous films were obtained by heat treatment at 400 • C for 2 h.

Characterization
The phase and crystal structures of the products were determined by X-ray diffraction (XRD) patterns, which were recorded with the X-ray diffractometer (the Philips X'Pert) with a 1D array detector using Cu Kα1 radiation (λ = 1.5406 Å). The morphology and structure of the product were observed by field emission scanning electron microscope (FE-SEM, FEI Sirion 200) and transmission electron microscope (TEM, JEOL JEM-2100). High-resolution transmission electron microscopic (HR-TEM) images and selected-area electron diffraction (SAED) patterns were acquired on a JEOL JEM-2100 transmission electron microscope at an operating voltage of 200 kV. The energy-dispersive X-ray spectroscopy (EDS) spots pattern scanning analysis was recorded by the TEM attachment. The X-ray photoelectron spectroscopy (XPS) analyses were carried out on a photoelectron spectrometer (ESCALAB Nanomaterials 2023, 13, 733 4 of 14 250XI) operated at an acceleration voltage of 15 kV and a current of 10 mA, and the binding energy was calibrated with reference to the C 1s binding energy 284.8 eV.

Gas-Sensing Tests
The sensor characterization was conducted by a STP4 intelligent gas-sensing analysis system (Nanjing Wisens Co. Ltd., Nanjing, China). The sensing performance of the fabricated devices was monitored in a sealed gas-sensing chamber at room temperature (25 • C) and 60% RH (relative humidity). Firstly, we injected enough liquid volatile organic compound into a fixed volume glass container to make it volatilize long enough to reach its saturated vapor concentration. Subsequently, a certain amount of target gas was extracted from the glass container through a micro syringe, and then it was injected into the test chamber. According to the volume ratio between the injected gas and the test chamber, the actual concentration of target gas in the test chamber was calculated. To illustrate the chemical gas-sensing ability of the ZnO/WO 3 films, we focused on four representative analytes: alcohols, aldehydes, benzenes, and ketones. These gases represent well-known toxic VOCs. The gas responses of the sensors are evaluated as R a /R g , where R a is the resistance of the sensor in air and R g is the resistance of the sensor in VOCs. The time taken by the sensors to achieve 90% of the total resistance change was defined as the response/recovery time (t) when exposed to the target gases or normal atmospheric environment.

Results and Discussion
Hexagonal packed MCC together with infiltrated precursor solutions were obtained on a curved ceramic tube by transfer of the MCC, which was self-assembled with PS spheres of 500 nm in diameter. After calcination, the bowl-shaped porous ordered ZnO/WO 3 films were obtained in situ due to the spherical geometry of the monolayer PS spheres. Subsequently, the structure, morphology, composition, and gas-sensing performances of the ZnO/WO 3 sensing films were systematically evaluated.
Despite the differences in chemical composition, it is rational for the template-assisted method to produce metal oxide sensing films with similar microstructures. Therefore, the 5% ZnO/WO 3 porous film was chosen for the following detailed demonstrations. Figure 3a shows the FE-SEM observation of the utilized MCC template prepared by the self-assembly process. It reveals that the PS spheres are hexagonally packed close to each other to form two-dimensional (2D) colloidal crystals with long-range order. The upper illustration of Figure 3a confirms its monolayer feature. After calcination in an air atmosphere, PS spheres were removed and the honeycomb ZnO/WO 3 composite porous film was readily obtained (Figure 3b). The ZnO/WO 3 nanocomposite sensing layer directly constructed on the substrate retains a large-scale ordered arrangement of nanopores, revealing a faultless template replication process. The porous structure makes the sensing layer have good mechanical stability. It is worth noting that the so-called "ordered porous" here refers to the overall honeycomb porous structure of the films, rather than the existence of many micropores or mesopores inside the films. The magnified observation (Figure 3c) illustrates that a single hexagonal aperture is about 500 nm. The monolayer structure is demonstrated by the cross-section SEM image in Figure 3d. After the annealing treatment to remove the PS template, the thickness of the films finally obtained is the radius size of the colloidal crystal (about 250 nm). Compared with the sensing films prepared by other brush-coated methods, the ultrathin sensitive films prepared by the in situ growth method are more uniform and stable, and the specific surface area is larger, which is conducive to the sensing response of gas. This ultra-thin, porous structure facilitates the diffusion of gas molecules from the surface to the interior of the film, thereby improving the sensing performance in terms of gas-sensitive response and response/recovery time. Despite the differences in chemical composition, it is rational for the template-assisted method to produce metal oxide sensing films with similar microstructures. Therefore, the 5% ZnO/WO3 porous film was chosen for the following detailed demonstrations. Figure 3a shows the FE-SEM observation of the utilized MCC template prepared by the self-assembly process. It reveals that the PS spheres are hexagonally packed close to each other to form two-dimensional (2D) colloidal crystals with long-range order. The upper illustration of Figure 3a confirms its monolayer feature. After calcination in an air atmosphere, PS spheres were removed and the honeycomb ZnO/WO3 composite porous film was readily obtained (Figure 3b). The ZnO/WO3 nanocomposite sensing layer directly constructed on the substrate retains a large-scale ordered arrangement of nanopores, revealing a faultless template replication process. The porous structure makes the sensing layer have good mechanical stability. It is worth noting that the so-called "ordered porous" here refers to the overall honeycomb porous structure of the films, rather than the existence of many micropores or mesopores inside the films. The magnified observation (Figure 3c) illustrates that a single hexagonal aperture is about 500 nm. The monolayer structure is demonstrated by the cross-section SEM image in Figure 3d. After the annealing treatment to remove the PS template, the thickness of the films finally obtained is the radius size of the colloidal crystal (about 250 nm). Compared with the sensing films prepared by other brush-coated methods, the ultrathin sensitive films prepared by the in situ growth method are more uniform and stable, and the specific surface area is larger, which is conducive to the sensing response of gas. This ultra-thin, porous structure facilitates the diffusion of gas molecules from the surface to the interior of the film, thereby improving the sensing performance in terms of gas-sensitive response and response/recovery time. The morphology and crystallinity of the porous films were characterized using transmission electron microscopy (TEM) technologies. Figure 4a shows that the particle size of the large-scale 5% ZnO/WO 3 porous film is 500 nm. In the imaging area, the whole porous film is tightly arranged with uniform pore size. Figure 4b-d show the high-resolution TEM (HR-TEM) images of WO 3 and ZnO, respectively. The spacing between adjacent fringes is about 0.263 nm and 0.375 nm, which belong to the (220) and (020) crystal planes of WO 3 , while the spacing between the neighboring fringes is about 0.247 nm and 0.281 nm, belonging to the (101) and the (100) crystal planes of hexagonal ZnO phase. Moreover, based on the energy-dispersive X-ray spectroscopy (EDS) elemental analysis, the O, W, and Zn elements were uniformly dispersed on the surface of the 5% ZnO/WO 3 porous film (Figure 4e-g). The EDS result shown in Figure 4h demonstrates that the peaks of O, Zn, and W can be clearly seen in the survey spectrum. The 5% ZnO/WO 3 sensor was thus obtained.
In order to further investigate the elemental composition and chemical states of each element, X-ray photoelectron spectra (XPS) measurements were conducted for the WO 3 and 5% ZnO/WO 3 sensing films ( Figure 5). They reveal that the WO 3 film is only composed of two elements, W and O, while the 5% ZnO/WO 3 film has an additional Zn element, which was consistent with the previous XRD and EDS results (Figure 5a). The high-resolution spectra of W 4f for both films (Figure 5b) show two distinct peaks located at 37.9 and 35.8 eV, which can be attributed to W 4f 5/2 and 4f 7/2 , respectively, suggesting the presence of W 6+ in the film matrixes [33]. In comparison with the W 4f spectrum of WO 3 , the 4f 5/2 and 4f 7/2 spin-orbit peaks of 5% ZnO/WO 3 shift to lower binding energies of 37.9 and 35.8 eV, indicating that an n-n heterojunction might exist at the interface of ZnO and WO 3 , and a part of the electrons might transfer. Furthermore, the spectrum of Zn 2p for the 5% ZnO/WO 3 film (Figure 5c) specifically shows two strong peaks located at 1045.1 and 1022.2 eV, which correspond to Zn 2p 1/2 and 2p 3/2 spin-orbit of the Zn 2+ chemical state, respectively. Meanwhile, for the high-resolution O 1s spectra, the binding energies at 530.3, 531.3, and 532.4 eV can be ascribed to the lattice oxygen (O L ), oxygen vacancy (O V ), and chemisorption oxygen (O C ), respectively [34]. O L was commonly considered as a non-active oxygen species, which did not participate in the sensing reaction with target molecules, while O V and O C are positively related to the amount of active oxygen species adsorbed on the surfaces. The proportion of varying oxygen species for two sensing materials is displayed in Table 1, which displays that the addition of ZnO increases the content of O V and O C to 43%, thereby providing more potential to improve the gas-sensing performances for the 5% ZnO/WO 3 sensors.  The morphology and crystallinity of the porous films were characterized using transmission electron microscopy (TEM) technologies. Figure 4a shows that the particle size of the large-scale 5% ZnO/WO3 porous film is 500 nm. In the imaging area, the whole porous film is tightly arranged with uniform pore size. Figure 4b-d show the high-resolution TEM (HR-TEM) images of WO3 and ZnO, respectively. The spacing between adjacent fringes is about 0.263 nm and 0.375 nm, which belong to the (220) and (020) crystal planes of WO3, while the spacing between the neighboring fringes is about 0.247 nm and 0.281 nm, belonging to the (101) and the (100) crystal planes of hexagonal ZnO phase. Moreover, based on the energy-dispersive X-ray spectroscopy (EDS) elemental analysis, the O, W, and Zn elements were uniformly dispersed on the surface of the 5% ZnO/WO3 porous film (Figure 4e-g). The EDS result shown in Figure 4h demonstrates that the peaks of O, Zn, and W can be clearly seen in the survey spectrum. The 5% ZnO/WO3 sensor was thus obtained.   In order to further investigate the elemental composition and chemical states of each element, X-ray photoelectron spectra (XPS) measurements were conducted for the WO3 and 5% ZnO/WO3 sensing films ( Figure 5). They reveal that the WO3 film is only composed of two elements, W and O, while the 5% ZnO/WO3 film has an additional Zn element, which was consistent with the previous XRD and EDS results (Figure 5a). The highresolution spectra of W 4f for both films (Figure 5b) show two distinct peaks located at 37.9 and 35.8 eV, which can be attributed to W 4f5/2 and 4f7/2, respectively, suggesting the presence of W 6+ in the film matrixes [33]. In comparison with the W 4f spectrum of WO3, the 4f5/2 and 4f7/2 spin-orbit peaks of 5% ZnO/WO3 shift to lower binding energies of 37.9 and 35.8 eV, indicating that an n-n heterojunction might exist at the interface of ZnO and WO3, and a part of the electrons might transfer. Furthermore, the spectrum of Zn 2p for the 5% ZnO/WO3 film (Figure 5c) specifically shows two strong peaks located at 1045.1 and 1022.2 eV, which correspond to Zn 2p1/2 and 2p3/2 spin-orbit of the Zn 2+ chemical state, respectively. Meanwhile, for the high-resolution O 1s spectra, the binding energies at 530.3, 531.3, and 532.4 eV can be ascribed to the lattice oxygen (OL), oxygen vacancy (OV), and chemisorption oxygen (OC), respectively [34]. OL was commonly considered as a nonactive oxygen species, which did not participate in the sensing reaction with target molecules, while OV and OC are positively related to the amount of active oxygen species adsorbed on the surfaces. The proportion of varying oxygen species for two sensing materials is displayed in Table 1, which displays that the addition of ZnO increases the content of OV and OC to 43%, thereby providing more potential to improve the gas-sensing performances for the 5% ZnO/WO3 sensors.

Gas-Sensing Performances
The VOC gas-sensing properties of the four sensors were investigated in detail. Herein, we use toluene as a typical target gas to evaluate the gas-response performances. On account of the importance of working temperature for metal oxide semiconductor gas sensors, we tested the response of sensors at different temperatures to obtain the optimal operation temperature. Figure 6 illustrates the responses of these sensors to 100 ppm toluene vapor in the temperature range of 100-300 • C. The results show that all the responses of these four sensors are the highest at about 300 • C. Even as the sensors' operating temperatures rise to 300 • C, their response tends to increase. This is consistent with that of traditional n-type semiconductor sensors for reducing gases. However, as continued temperature increase will result in a high energy consumption, which does not meet the actual realtime detection requirements, 300 • C is selected as the optimal operation temperature of those sensors in present work. In addition, the combination of ZnO and WO 3 leads to significant enhancement in sensing responses to toluene compared with the pure WO 3 sensor in the temperature range of 100 • C to 300 • C. Simultaneously, the 5% ZnO/WO 3 sensor demonstrates the best response performance over all temperature ranges, and for excess addition of ZnO (the 10% ZnO/WO 3 sensor), the gas-sensitive performances are remarkably decreased [30,35,36]. It can be seen from the XRD pattern ( Figure 2) that a new material ZnWO 4 appears in the 10% ZnO/WO 3 composite system. The formation of ZnWO 4 may be the main reason for the decrease of gas sensitivity of 10% ZnO/WO 3 . The gas sensitivity of the ZnO/WO 3 composite system will be improved with the increase of ZnO content, and the gas sensitivity will reach the best when the molar content of ZnO is 5%. From the subsequent gas-sensing enhancement mechanism, we can know that there are n-n heterostructures in the ZnO/WO 3 composite system, which will significantly improve the gas-sensing performance. The production of ZnWO 4 will break the original best composite system, thus reducing the gas sensitivity of 10% ZnO/WO 3 . The same change trends can be observed for typical single-cycle response curves of the four sensors at the optimal operating temperature of 300 • C. This emphasizes the significant importance of the appropriate combination of ZnO for WO 3 sensors. In addition, a response of R a /R g = 68 was obtained for the 5% ZnO/WO 3 sensor towards 100 ppm toluene with a response and recovery time of 0.66 s and 2.5 s, respectively. An assessment of sensing performances towards toluene vapor in the literature is overviewed in Table 2, which confirms a significant advance of the as-prepared ZnO/WO 3 sensor in present work. The response/recovery times of the ZnO/WO 3 sensor for toluene are all less than 3 s, which is much less than that of other toluene-sensing materials [37][38][39]. Such sensitive and fast responses are beneficial to the trace and real-time detection of toxic VOCs for widespread household air quality-monitoring applications.
Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 15 excess addition of ZnO (the 10% ZnO/WO3 sensor), the gas-sensitive performances are remarkably decreased [30,35,36]. It can be seen from the XRD pattern ( Figure 2) that a new material ZnWO4 appears in the 10% ZnO/WO3 composite system. The formation of ZnWO4 may be the main reason for the decrease of gas sensitivity of 10% ZnO/WO3. The gas sensitivity of the ZnO/WO3 composite system will be improved with the increase of ZnO content, and the gas sensitivity will reach the best when the molar content of ZnO is 5%. From the subsequent gas-sensing enhancement mechanism, we can know that there are n-n heterostructures in the ZnO/WO3 composite system, which will significantly improve the gas-sensing performance. The production of ZnWO4 will break the original best composite system, thus reducing the gas sensitivity of 10% ZnO/WO3. The same change trends can be observed for typical single-cycle response curves of the four sensors at the optimal operating temperature of 300 °C. This emphasizes the significant importance of the appropriate combination of ZnO for WO3 sensors. In addition, a response of Ra/Rg = 68 was obtained for the 5% ZnO/WO3 sensor towards 100 ppm toluene with a response and recovery time of 0.66 s and 2.5 s, respectively. An assessment of sensing performances towards toluene vapor in the literature is overviewed in Table 2, which confirms a significant advance of the as-prepared ZnO/WO3 sensor in present work. The response/recovery times of the ZnO/WO3 sensor for toluene are all less than 3 s, which is much less than that of other toluene-sensing materials [37][38][39]. Such sensitive and fast responses are beneficial to the trace and real-time detection of toxic VOCs for widespread household air quality-monitoring applications.    The dynamic response-recovery curves of the four sensors to toluene vapor with stepped concentrations were systematically evaluated at 300 • C, as demonstrated in Figure 7a. Although the responses of all these gas sensors steadily increased with increasing toluene concentration, the 5% ZnO/WO 3 sensor displayed the most significant sensitive performance. Despite having the strongest response to the target gas, the 5% ZnO/WO 3 sensor showed much smoother fluctuations of the electrical signal at the equilibrium state, revealing a perfect signal stability. Moreover, the composite sensor had a limit of detection (LOD) down to 0.1 ppm, while pure WO 3 had an LOD of 10 ppm. Meanwhile, it was difficult for other sensing materials ( Table 2) to achieve such a low detection limit. The relationship between the response of the ZnO/WO 3 sensor and the concentration of toluene was further studied. As exhibited in Figure 7b, the response values followed an approximately linear increase with the increase of toluene concentration in the range of 0.1−200 ppm, demonstrating that the 5% ZnO/WO 3 sensor can work as a toluene vapor sensor in a wide linear range. The dynamic response-recovery curves of the four sensors to toluene vapor with stepped concentrations were systematically evaluated at 300 °C, as demonstrated in Figure 7a. Although the responses of all these gas sensors steadily increased with increasing toluene concentration, the 5% ZnO/WO3 sensor displayed the most significant sensitive performance. Despite having the strongest response to the target gas, the 5% ZnO/WO3 sensor showed much smoother fluctuations of the electrical signal at the equilibrium state, revealing a perfect signal stability. Moreover, the composite sensor had a limit of detection (LOD) down to 0.1 ppm, while pure WO3 had an LOD of 10 ppm. Meanwhile, it was difficult for other sensing materials (Table 2) to achieve such a low detection limit. The relationship between the response of the ZnO/WO3 sensor and the concentration of toluene was further studied. As exhibited in Figure 7b, the response values followed an approximately linear increase with the increase of toluene concentration in the range of 0.1−200 ppm, demonstrating that the 5% ZnO/WO3 sensor can work as a toluene vapor sensor in a wide linear range. As we focus on assessing the air quality by monitoring concentrations of TVOC (total volatile organic compounds), it is expected that the sensor should have simultaneous response towards multiple organic solvents that commonly exist in daily life while suppressing the sensing performances to conventional flammable or toxic gases. Therefore, in addition to toluene, we also chose formaldehyde, ethanol, and acetone as representative VOC targets, and several reducing/oxidizing gas molecules such as methane, hydrogen, carbon oxide, nitric oxide, and nitric dioxide as interfering gases. Figure 8a demonstrates As we focus on assessing the air quality by monitoring concentrations of TVOC (total volatile organic compounds), it is expected that the sensor should have simultaneous response towards multiple organic solvents that commonly exist in daily life while suppressing the sensing performances to conventional flammable or toxic gases. Therefore, in addition to toluene, we also chose formaldehyde, ethanol, and acetone as representative VOC targets, and several reducing/oxidizing gas molecules such as methane, hydrogen, carbon oxide, nitric oxide, and nitric dioxide as interfering gases. Figure 8a demonstrates that the 5% ZnO/WO 3 sensor shows sensitive and rapid response upon exposure of those individual VOC targets with concentrations of 100 ppm at 300 • C, while no obvious response can be observed for all those interfering gases. Furthermore, in comparison with the WO 3 sensor, the 5% ZnO/WO 3 sensor shows significant improvement in responses to those organic vapors, revealing a perfect cross-sensitivity towards VOC targets (Figure 8b). This feature facilitates accurate recognition of the characteristic gaseous constituents in complex environmental conditions, thereby providing more reliable diagnostic data for air quality assessment. Sensing stability is one of the key criteria to evaluate whether the gas sensor can used for practical applications. A good stability requires that the sensor can repeate respond to the target gas of a specific concentration without significant response decrea Hence, the response reproducibility of the four sensors to repeated exposure of tolu vapor were experimentally explored. Figure 9a demonstrates typical sensing respon for five repeatable circles to 100 ppm toluene vapor at the operating temperature of 300 It depicts that although the RSD (relative standard deviation) values of the correspond responses were all less than 10%, the 5% ZnO/WO3 sensor showed the best signal rep ducibility (RSD = 3.49%) among the four sensors, which is of great significance for relia quantitative gas monitoring. Furthermore, the long-term stability of the 5% ZnO/WO3 s sor to toluene with different concentrations was also measured, as demonstrated in Fig  9b. It reveals that the sensor exhibited nearly constant responses (RSD < 2%) to 1, 5, 20, and 100 ppm toluene with a long period of intermittent tests, confirming a perfect stabi of the as-developed ZnO/WO3 sensors for VOC detection. Sensing stability is one of the key criteria to evaluate whether the gas sensor can be used for practical applications. A good stability requires that the sensor can repeatedly respond to the target gas of a specific concentration without significant response decreases. Hence, the response reproducibility of the four sensors to repeated exposure of toluene vapor were experimentally explored. Figure 9a demonstrates typical sensing responses for five repeatable circles to 100 ppm toluene vapor at the operating temperature of 300 • C. It depicts that although the RSD (relative standard deviation) values of the corresponding responses were all less than 10%, the 5% ZnO/WO 3 sensor showed the best signal reproducibility (RSD = 3.49%) among the four sensors, which is of great significance for reliable quantitative gas monitoring. Furthermore, the long-term stability of the 5% ZnO/WO 3 sensor to toluene with different concentrations was also measured, as demonstrated in Figure 9b. It reveals that the sensor exhibited nearly constant responses (RSD < 2%) to 1, 5, 20, 50, and 100 ppm toluene with a long period of intermittent tests, confirming a perfect stability of the as-developed ZnO/WO 3 sensors for VOC detection.
It depicts that although the RSD (relative standard deviation) values of the corresponding responses were all less than 10%, the 5% ZnO/WO3 sensor showed the best signal reproducibility (RSD = 3.49%) among the four sensors, which is of great significance for reliable quantitative gas monitoring. Furthermore, the long-term stability of the 5% ZnO/WO3 sensor to toluene with different concentrations was also measured, as demonstrated in Figure  9b. It reveals that the sensor exhibited nearly constant responses (RSD < 2%) to 1, 5, 20, 50, and 100 ppm toluene with a long period of intermittent tests, confirming a perfect stability of the as-developed ZnO/WO3 sensors for VOC detection.

Gas-Sensing Mechanism
Gas sensing is known to be directly related to the adsorption of active species and subsequent surface reactions [30]. At present, the most widely accepted sensing mechanism in the literature is based on the redox reaction between the pre-adsorbed oxygen species from the environment and the following target molecules, which occurs on the surface of sensing materials [44][45][46]. When an n-type semiconducting matrix is exposed to the air (Figure 10a), oxygen molecules are adsorbed on the surface and subsequently capture free electrons from the conductance bands of the semiconductor, forming negatively charged surface oxygen species (O 2 − , O − , O 2− ) [36]. As a result, an electron depletion layer (EDL) forms on the surface domains, and the sensing matrix is in a high-resistance state. The corresponding reaction processes are shown in Equations (1)-(4) [47]:

Gas-Sensing Mechanism
Gas sensing is known to be directly related to the adsorption of active species and subsequent surface reactions [30]. At present, the most widely accepted sensing mechanism in the literature is based on the redox reaction between the pre-adsorbed oxygen species from the environment and the following target molecules, which occurs on the surface of sensing materials [44][45][46]. When an n-type semiconducting matrix is exposed to the air (Figure 10a), oxygen molecules are adsorbed on the surface and subsequently capture free electrons from the conductance bands of the semiconductor, forming negatively charged surface oxygen species (O2 − , O − , O 2− ) [36]. As a result, an electron depletion layer (EDL) forms on the surface domains, and the sensing matrix is in a high-resistance state. The corresponding reaction processes are shown in Equations (1)-(4) [47]: − ( ) + − ↔ 2− (T > 300 °C) (4) Figure 10. Schematic gas sensing diagram of (a) pure WO3 − and (b) ZnO/WO3 nanocomposite-based sensors exposed to air and VOC, respectively. The space-charge layer model is used to explain the sensing mechanism.
Furthermore, it has been reported that the formation of heterogeneous structures between two metal oxide semiconductors can significantly improve the adsorption of O2 due to the existence of electronic effects [35,48,49]. Thus, more O2 adsorption sites exist on the surface of ZnO/WO3 nanocomposite films due to the formed n-n heterojunction. Accordingly, compared with pure WO3, the ZnO/WO3 composite possesses a thicker depletion layer on the surface and produces a larger bulk resistance (Figures 10b and S1). When the Figure 10. Schematic gas sensing diagram of (a) pure WO 3 − and (b) ZnO/WO 3 nanocomposite-based sensors exposed to air and VOC, respectively. The space-charge layer model is used to explain the sensing mechanism.
Furthermore, it has been reported that the formation of heterogeneous structures between two metal oxide semiconductors can significantly improve the adsorption of O 2 due to the existence of electronic effects [35,48,49]. Thus, more O 2 adsorption sites exist on the surface of ZnO/WO 3 nanocomposite films due to the formed n-n heterojunction. Accordingly, compared with pure WO 3 , the ZnO/WO 3 composite possesses a thicker depletion layer on the surface and produces a larger bulk resistance (Figures 10b and S1). When the sensing matrix is exposed to the reducing toluene vapor, the toluene molecules undergo a redox reaction with the surface-adsorbed oxygen species and release electrons into the interior of sensing material. For the ZnO/WO 3 composite, it will result in a more significant decrease in the bulk resistance and more active oxygen species on the surface, which produce enhanced gas-sensing properties [50].

Conclusions
In summary, we proposed a sacrificial template-based in situ approach for the facile fabrication of ZnO/WO 3 ordered nanoporous sensing films on commercial ceramic tubes with curved and rough surfaces. The as-fabricated nanocomposite films possess twodimensional honeycomb-like porous features with 250 nm in thickness and large-scale structural consistency. Such features endue the nanocomposite sensors with sensitive responses, fast response/recovery, and repeatable detection performances. Typically, for the 5% ZnO/WO 3 composite sensor, a detectable concentration of 0.1 ppm with a second-level response time and perfect signal reproducibility (RSD = 3.49%) was obtained. Furthermore, we verified that the combination of two sensitive materials would generate abundant heterojunctions inside and on the surfaces of the sensing matrixes. This significantly increases the thickness of the charge depletion layer and the adsorption of active oxygen species on the surface, which is conducive to increasing the cross-sensitivity of the gas sensor to VOCs while suppressing the response to other kinds of interfering gases. The proposed in situ strategy for the preparation of nanocomposite sensing films is not only conducive to optimizing the performance of gas sensors through the regulation of nanostructures and chemical composition, but also provides a technical route for the development of cross-sensitive gas sensors.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13040733/s1, Figure S1: The initial resistance of the four sensors in air atmosphere at 300 • C.